THE MILKY WAY GALAXY

The milky way is the name given to the faint band of light that stretches across the night sky. This light comes from stars and nebulae in our galaxy, known as the milky way galaxy or simply as ”the galaxy”. The galaxy is shaped like a spiral, with a dense central bulge that is encircle by four arms spiraling outwards and surrounded by a less dense halo. We cannot see the spiral shape because the solar system is in one of the spiral arms, the Orion arms(also called the local arm). From our position, the center of the galaxy is completely obscured by dust clouds; as a result, optical maps give only limited view of the galaxy. However, most complete picture can be obtained by studying radio, infra-red, and other radiations. The central bulge of the galaxy is a relatively small, dense sphere that contains mainly older red and yellow stars. The halo is a less dense region  in which the oldest stars are situated; some of  these stars may be as old as the galaxy itself (possibly 15 billion years). The spiral arms contain mainly hot, young, blue star, as well as nebula (clouds of dust and gas inside which stars are born). The galaxy is vast, about 100,000 light years across(a light year is about 9,460 billion kilometres ); in comparison, the solar system seems small, at about 12 light hours across(about 13 billion kilometres ). The entire galaxy is rotating in space, although the inner travels faster than those further out. The sun, which is about two-thirds out from the center, completes one lap of the galaxy about every 220 million years.

NEBULA AND STAR CLUSTERS

A nebula is the cloud of dust and gas inside a galaxy.  Nebula become visible if the gas glows, or if the cloud reflects starlight or obscure light from more distant objects. Emission nebulae shine because their gas emits light when it is stimulated by radiation from hot young stars. Reflection nebula shine because their dust reflects light from stars in or around the nebula. Dark nebulae appear as silhouettes because they block out light from shining nebulae or star behind them. Two types of nebula are associated with the dying stars: planetary nebula and supernova remnants. Both consist of expanding shell of gas that were once the outer layers of a star. A planetary nebula is a gas shell drifting away from a dying stellar core. A supernova remnant is a gas shell moving away from a stellar core at great speed following a violent explosion called a supernova. Stars are often found in groups known as clusters. Open clusters are loose groups of a thousand young stars that were born in the same cloud and are drifting apart. Globular clusters are densely packed, roughly spherical groups of  hundreds of thousands of older stars.

PLANETARY  NEBULAE

Planetary nebulae is a stage of star’s death. A planetary nebula, singular for nebulae is a shell of illuminated gas surrounding certain dying stars. Planetary nebulae have central stars and these stars were once lower to medium sized stars, like that of our sun, which eventually become a giant star and will soon become a white dwarf. As adult stars, the stars that make planetary nebulae burn hydrogen for fuel and leave helium ash as a byproduct. The hydrogen eventually runs out but these stars are massive enough to force the helium to be used as a secondary fuel thereafter. The helium is burned to a carbon  core as a byproduct. The helium eventually  begins to runs out as well. But unfortunately these stars are not massive enough to force the ignition of carbon as a tertiary fuel and they begin to die instead. As our stars becomes red giants and run out of helium fuel, they reignite the helium energy shells and increase the luminosity  called thermal pulse.ie, a thermal pulse is a temporary re-ignition of helium and increase of luminosity in a dying star. Luminosity is the total energy a star radiates out in one second. It is during the thermal pause, that the star’s outer gaseous layer were separate from its core, one that is now made up of carbon. In a sense, as the old star momentarily hits its gas pedal to use its remaining helium for energy production. During this time, it’s very hot inner core becomes exposed. The surface temperature is so high that the core will emit ultra violet radiations which is strong enough to ionize these shells of previously ejected gas. It is this that causes the gases to glow and produce all sort of amazing colors.

SUPERNOVA

A supernova is a massive explosion generated by a dying star. A star in our galaxy goes supernova approximately once every one hundred years. The explosion holds matter in the space that shines as brightly as an entire galaxy for short period. Stars are giant nuclear reactors producing energy by fusing hydrogen in to helium. Eventually however when  the star is out of hydrogen to fuse, the helium will fuse to heavier elements including iron. The star’s core shrinks and the outer layer expand, creating a red giant that consumes the surrounding planets. When the star runs out of helium, the star becomes a white dwarf and slowly fades away. When a large star depletes its helium, its core collapse within seconds, causing temperature of one hundred billion degrees. The outer layer of the star collapse as well. This massive explosion is known as supernova. Supernova generate enough energy to fuse elements heavier than iron. Gold, silver, uranium and other elements with higher atomic numbers are created in the supernovas. Then the star becomes a dense neutron star or a black hole. After the supernova, materials expelled  in the explosion forms a nebulae which has a massive inner stellar cloud of gas and dust. Over million years ,the gravity pulls nebulae materials together in to a dense hot core called a protostar. Protostars eventually become newborn stars.